The invention relates to a method for DC drift-free polarization transformation and DC drift-free polarization transformers.
German patent application DE 19 830 990.2 describes a polarization transformer/compensator which is implemented on a birefringent substrate material. This is a development from a polarization transformer which has been described in IEEE Journal of Quantum Electronics, Vol. QE-18, No. 4, April 1982, pages 767-71.
That configuration consists of a lithium niobate chip, which carries electrodes at its surface. An isolating buffer layer is usually applied between the substrate and the electrodes which prevents the attenuation of the optical signal if metal electrodes are used. That configuration is subject to the so-called DC drift problem which is caused by signals of constant polarity. The DC drift problem is caused by the fact that buffer layers and electrodes exhibit different ratios of dielectricity constant to conductivity. Due to the dielectric properties of substrate and buffer layer the initial potential distribution is initially given by the electrostatic field. As time goes by it will be modified and will converge into a potential distribution given by conductivities of substrate and buffer layer. Although the voltage applied to the electrodes stays constant the field inside the lithium niobate chip changes due to the new potential distribution, in particular also inside the optical waveguide which means not the desired but another electro optic effect will result.
Another, very pernicious reason for DC drift is believed to be due to the fact that high incident optical power, but also usual power applied over longer time periods, can produce charge carrier pairs due to absorption. If a DC voltage and hence an electrical field is present between the electrodes these charge carrier pairs will be separated by the electrical field. This results in a weakened electrical field. Therefore higher and higher voltages are needed in time to achieve the desired polarization transformations. This either exceeds the capabilities of the chosen voltage sources or will result in discharges between the electrodes. It has to be considered that a high-performance polarization transformer of the mentioned kind may require fairly high voltages of up to about 100V. DC drift can therefore restrict or even prevent the due function of a compensator. DC drift occurs also in almost all other lithium niobate components (polarization transformers) designed for polarization transformation or PMD compensation, for which a solution of the drift problem is also sought therefore.
Up to now it has been attempted to solve the problem by improved technology with improved balance of dielectricity constant and conductivity of the buffer layer, by a crystal with lower loss, and by other means. Even for lithium niobate intensity modulators which are operated only with small voltages this seems to have been managed only partly.
It is accordingly an object of the invention to provide a method for DC drift-free polarization transformation and DC drift-free polarization transformers, which overcomes the above-mentioned disadvantages of the heretofore-known devices and methods of this general type and which offers a relatively simple method for avoiding DC drift in polarization transformers and PMD compensators.
With the foregoing and other objects in view there is provided, in accordance with the invention, a method for DC drift-free polarization transformation or polarization mode dispersion compensation by a polarization transformer having a waveguide and control electrodes, which comprises:
feeding control voltages to the control electrodes for changing a state-of-polarization or a polarization mode dispersion of an optical signal; and
adjusting the control voltages to have substantially vanishing DC components.
In accordance with an added feature of the invention, the method further comprises applying a differential phase modulation of two orthogonal principal states-of-polarization of the optical signal which are identical to the principal states-of-polarization of a polarization transformer at a signal input with a continuous differential phase shift chosen such that a temporal average of a cosine function thereof and a temporal average of sine function thereof vanish at least approximately.
The solution of the problem lies in the use of DC component-free driving voltages. The architecture of the polarization transformer and the driving voltages are chosen such that the function of the polarization transformer is not compromised. A large number of embodiments exist, all of which are based on the same principle.
In an embodiment of the invention it is particularly advantageous to apply a differential phase modulation between the two orthogonally polarized principal states-of-polarization (PSP) of the polarization transformer. If the principal states-of-polarization are TE- and TM waves (TExe2x80x94transverse electric; TMxe2x80x94transverse magnetic), a differential TE-TM phase modulator at the input of a compensator can be provided.
This has the advantage that a differential TE-TM phase modulation of the incident lightwave is generated. For a suitably chosen phase modulation the following TE-TM mode converter cells can be driven by DC voltage-free signals.
It is particularly advantageous to drive the TE-TM phase modulator by a triangular voltage of low frequency.
This allows the TE-TM converter electrodes to be driven by DC component-free cosine or sine voltages (to be more precise: usually by subsequent parts of sine voltages applied in alternately forward and backward direction of the angle argument). These converter voltages are impressed as cosine or sine functions while the actual task of PMD compensation results only from changes of amplitude and phase. Since the triangular voltage can also be chosen free of DC component there is no DC drift in the differential TE-TM phase modulator either in this case; however, it would not be detrimental there anyway.
At least a part of the converter driving voltages, just like the driving voltage of the phase modulators or mode converters, can be generated by a control unit.
Alternatively to the use of a TE-TM phase modulator a corresponding differential TE-TM phase modulation can also be generated by one or a few TE-TM converters which are preferably situated in the input section of the chip. For this purpose those converter cells which are not situated in the input section of the chip receive DC voltage-free driving voltages while the driving voltages of the first converter cells are generated by the compensatory control system.
Another possibility to generate a differential TE-TM phase modulation consists in adding at least one converter cell which just like the first converter cell used for PMD compensation is operated with special DC component-free driving voltages.
The use of a second TE-TM phase modulator can be advantageous in order to achieve an output state-of-polarization that is independent of time-dependent changes of the driving voltage. The same is valid for the other described ways for realization or substitution of a TE-TM phase modulator.
The methods described for TE and TM waves as principal states-of-polarization can also be used for other, e.g., circular principal states-of-polarization.
However, as an architecture of a polarization transformer which allows for a DC component-free choice of driving voltages without compromising the function of the polarization transformer, the enhancement of polarization transformers by additional control elements such as differential phase modulators, mode converters or additional converter cells is likewise possible.
With the above and other objects in view there is provided, in accordance with the invention, a polarization transformer for DC drift-free polarization transformation or polarization mode dispersion compensation, comprising:
a chip having a waveguide with an input;
a plurality of comb-shaped mode converter electrodes disposed perpendicularly to the waveguide, the mode converter electrodes receiving control voltages for changing a state-of-polarization or a PMD of an optical signal;
a comb-shaped ground electrode disposed in vicinity of the mode converter electrodes; and
a device selected from the group consisting of a differential phase modulator and a mode converter at the input.
In accordance with an added feature of the invention, another phase modulator or mode converter is disposed at the output.
In accordance with an additional feature of the invention, at least one converter cell is defined on the chip, the converter cell comprising several comb-shaped converter electrodes running perpendicular to the waveguide, and a comb-shaped ground electrode.
In accordance with another feature of the invention, the converter cells include TE-TM converter cells having two mode converter electrodes with varying spaces between mutually adjacent mode converter electrodes.
In accordance with a preferred embodiment of the invention, the polarization transformer is implemented on a lithium niobate chip with at least approximate Y propagation. Preferably, the chip has an at least approximate X cut or Z cut.
In accordance with a further feature of the invention, a differential phase shifter comprises two electrodes running on either side of the waveguide.
With the above and other objects in view there is also provided, in accordance with the invention, a polarization transformer for DC drift-free polarization transformation or polarization mode dispersion compensation, comprising:
a chip having a chip surface, a waveguide with an input, and a plurality of comb-shaped mode converter electrodes receiving control voltages for changing a state-of-polarization or a PMD of an optical signal;
a device selected from the group consisting of a differential phase modulator and a mode converter at the input; and
electrodes on two sides of the waveguide for generating electrical fields along the chip surface running perpendicular to the waveguide.
There is also provided, in accordance with the invention, a polarization transformer for DC drift-free polarization transformation or polarization mode dispersion compensation, comprising:
a chip having a waveguide with an input;
a plurality of mode converter electrodes receiving driving voltages for changing the state-of-polarization or the PMD of an optical signal; and
a device selected from the group consisting of a differential phase modulator and a mode converter at the input.
In accordance with again an added feature of the invention, a further polarization control element operates substantially as a quarterwave plate and has eigenmodes allowing for a transformation of a circular state-of-polarization into a principal state-of-polarization of a polarization-maintaining optical fiber connected to the polarization transformer.
In accordance with again an additional feature of the invention, at least one polarization-maintaining optical fiber is connected to the chip at a defined connection point and having principal states-of-polarization enclosing angles of xc2x145xc2x0 with respect to a chip surface of the chip, and a polarization control element disposed at the connection point and having substantially horizontal and vertical eigenmodes.
In accordance with again a concomitant feature of the invention, the differential phase modulator is a circular retarder.
With the above objects in view there is further provided, in accordance with the invention, a polarization transformer for DC drift-free polarization transformation or polarization mode dispersion compensation, comprising:
a chip having a waveguide conducting an optical signal;
at least one first polarization transformer for changing a state-of-polarization or a PMD of the optical signal;
at least one second polarization transformer adapted to alternately and at least partly take over a function of the at least one first polarization transformer and to be driven by driving signals opposed to taking over the function.
Other features which are considered as characteristic for the invention are set forth in the appended claims.
Although the invention is illustrated and described herein as embodied in method for DC drift-free polarization transformation and DC drift-free polarization transformer, it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims.